Selective predation may be an important proximate cause of the success or failure of invader species. Gammarus roeseli is an invasive amphipod, for which the causes of establishment in rivers where the native species, Gammarus pulex, predominates remain unstudied. Freshwater amphipods are important prey for numerous fish predators, but empirical evidence of lower predation rates on exotic prey is scarce. In laboratory experiments, we compared the susceptibility of G. pulex and G. roeseli to fish predation, determined the mechanisms influencing prey selection, and studied the interaction between behavioural and morphological defences. Fish predators (brown trout, Salmo trutta fario) preyed selectively on G. pulex, but not because of differences in attack or capture probability. The presence of spines in G. roeseli appeared to contribute to its underpredation. Differential prey selection in this case might therefore have resulted from the trout's reaction to an adverse stimulus. We found no significant difference in antipredator behaviour between G. pulex and G. roeseli. General behavioural differences were nevertheless found between species, with G. roeseli spending more time under shelters than G. pulex. However, microcosm experiments suggested that this difference was not important for differential predation. Antipredator behaviour may nevertheless be important for G. roeseli against other predators less sensitive to spines.